Condenser with temperature compensation



()ch 1, 3935; s. w. SEELEY CONDENSER WITH TEMPERATURE COMPENSATION FiledJune 21, 1933 W 4 u 4% m a. 2 34 .vi a I sun EP Rim W V. n d i a mf w .IN. M m n J m m w 5 Q Patented Oct. 1, 1935 UNITED STATES PATENT OFFICEStuart W. Seeley; Jackson,

Mich., assignor, by

mesne assignments, to Radio Corporation of America, New York, N.

Delaware Y., a corporation of Application June 21, 1933, Serial No.676,815

2 Claims.

This invention relates to electrical condensers. The capacity of acondenser varies directly as its area and inversely as the distancebetween the plates. The equation of a condenser is:

where k is a constant A is the plate area and D is the distance betweenthe plates of opposite 1o charge. If the temperature of a condenserincreases, the area A is increased due to the coeflicient of expansionof the plates. This change in area from temperature variation is afunction of the square of the eoefficient of expansion 15 of theparticular metal used. As the temperature changes the distance betweenthe stator plates also changes due to the expansion or contraction ofthe spacing washers. The distance D varies directly as the coefiicientof expansion.

20 The coefficient of expansion therefore appears as the square of thenumerator of the right hand side of the above equation and as alinearfunction in the denominator. Increase of temperature accordinglyincreases the capacity of the 25 condenser as a linear function of thecoefficient of expansion of the metal used.

In many instances it is quite material to maintain a constant capacity,for example, in broadcast and other transmission systems.

30 It is an object of this invention to produce a condenser that hasconstant capacity regardless of temperature changes as will more fullyappear in the annexed specification, reference being had to the drawing,in which:

35 Fig. 1 is a plan of a condenser embodying the invention.

Fig. 2 is an end view of the condenser.

Fig. 3 is a fragmentary view showing a modifled rotor shaft.

Fig. 4 is a modified form of rotor shaft.

The condenser may be of the type having a rotor shaft grounded onconducting end plates or of the type having the end plates made ofinsulation material, but for purposes of illustra- 45 tion I have shownthe latter type. The stator consists of stator plates I, 2 and 3separated by bushings l on clamping rods 5 and a compensating statorplate 6 spaced from the other stator plates by long bushings l whichmay, if desired,

5 be, of a plurality of shorter bushings such as 4. End bushings 8, 9,separate the end plates l0 and II from the stator plates. Nuts l2 on therods clamp the assembly of main and compensating stator plates rigidlytogether.

5 The rotor shaft H is made of material that has greater temperaturecoefiiciency of expansion than the bushings l and is journalled inbush== ings i5 in the end plates l0 and H which are of insulation. Onone end. of the shaft I4 is manual knob 16 held by pin l6 and on theother end is B a pair of nuts H and a spring washer l8. At normaltemperatures one of the nuts I! would be threaded against washer l8 soas to permit this washer to yield during contraction and expansion ofthe shaft I4 as the room temperature 10 changes. The other nut, I? wouldbe jammed against the first mentioned nut so as to lock it in position.On this shaft is mounted main rotor plates l8 and compensating rotorplate is.

At normal temperature, say 20 centigrade, the condenser with a givensetting of rotor plates would have a definite capacity. If thetemperature should increase with this same setting the stator and rotorplates would increase in area as some function of the coefficient ofexpansion of the material from which they are made. With. this sameincrease in temperature the rotor shaft I4 will increase in length fromend plate l0 more than the bushings 4, l and 8 that separate the statorplates from such end plate. Consequently the distance between the rotorplate l9 and the stator plate 6 will vary quite materially and thusoffset the increase in capacity due to the increase in area of theplates.

The rotor and stator plates may be made of aluminum, brass or other goodconducting metal, as is customary, but' the bushings 1 should be made ofsome metal with low coefficient of expansion and the rotor shaft M of ametal of higher coefficient of expansion. The bushings may be made ofiron which has a coeflicient of expansion in the neighborhood of 9x10-and the rotor shaft may be made of bronze'which has, say, a constituencyof 93.5% of copper and 6.5% of tin. This gives a coefficient ofexpansion of 40 about 36 x 10*. By adjusting the length of the shaft Mthe compensating rotor plate Hi can be made to have such distance fromits compensating rotor plate 6 as to exactly offset the increase of theeffective plate area and thus maintain the capacity constant with changein temperature.

It is advisable to copper-plate the iron bushings or to provide a copperjumper to lower the resistance between plates. When jumpers are usedbushings 7 of low expansion non-conduct-= ing materials may be used.

Of course the expansion of bushings 4 oppose the increase in capacitydue to area increase and this will be added to the effect of thecompensat- 5 ing plates. For this reason the bushings may be made ofmaterial of high coefiicient of expansion.

The axial movement of rotor plates l8 due to expansion does notmaterially affect the capacity but this change also can be compensatedas it is additive to the change due to area increase with centrallylocated plates and the condenser design can take this into account.

It of course is advisable, if otherwise satisfactory, to choose metalfor the plates that has a low coeflicient of expansion. Copper has aboutas low a coefficient of expansion as metals that are otherwisesatisfactory, but if iron plates are copper-plated they may be used andadvantage obtained of the relatively low coeflicient of expansion ofthis metal. The alloy Invar has a very low coefficient of expansion.This is an alloy of nickel and iron and it has recently been shown thatby varying the proportions of these metals one can obtain an alloy thathas practically zero coefficient of expansion. Invar bushings of thisproportion, copper-plated, if desired, may be used on the shafts of thecondenser so as to make a greater variation in spacing between thecompensating plates l9 and 6 as the temperature varies. Carbon is also amaterial with low coefficient of expansion and copper plated bushings Imay likewise be used on the clamping rods 5 rotor shaft ll may be madeof some non-conducting material with high coefiicient of expansion withcables 20 to connect the rotor plates together electrically, as shown inFig. 3.

Hard rubber may be used for thispurpose. It has a coefllcient ofexpansion in the neighborhood of 80 x 10-. Celluloid would besatisfactory also. It has a still higher coefficient of expansion in theneighborhood of 109 x 10- a In Fig. 4 I have illustrated a modificationin which the stator spaces and the main rotor shaft are both of lowexpansion material and the compensating shaft is of high expansion. Inthis figure similar reference characters are used for similar parts inthe other figures. The spacers I, rods 5 and shaft II havelowcoefficients of expansion. There is therefore minimum change in thespaces D between the main rotor and sta- Where a higher compensation isneeded the tor plates. The shaft l4" passes freely through the tubularshaft l4 and both are fastened to the knob "5 by the pin IS. The reducedfree end of shaft I4" is journaled in the bushing of the end plate II.

In the modification of Fig. 4 if the temperature should increase thecompensating shaft N" would expand more rapidly than the shaft i4,

bushings 4, and the distance between the auxiliary rotor plate l9 andthe common stator plate 3 would increase. Consequently, the increase ofcapacity due to the increase of area in all the plates would becounteracted by the increased separation between the rotor plate 19 andits associated stator plate 3. With this arrangement one is able toobtain a shorter condenser than in the type of Fig. 1 and in additionthe main rotor plates are maintained midway between the stator platesfor all practical purposes.

While I have described the rotor shaft as hav ing the higher coefficientof expansion it will be apparent that the stator bushings or otherstator support can be made to have the greater coefficient with the sameresult if positioned outside the rotor plate IQ of Fig. 3.

Having described my invention, what I claim 1. An electrical condenserunit consisting of a frame, plates secured to said frame, a rotatableshaft of low coefficient of expansion, an additional plate secured tosaid shaft in spaced relation to one of the first mentioned plates, ashaft of high coefiicient of expansion fastened to the first shaft and acompensating plate secured to the end of the second shaft and spacedoutside of another of said first mentioned plates.

2. An electrical condenser unit consisting of a frame, a plurality ofstator plates secured to said frame, a shaft rotatingly secured to saidframe, a plurality of rotor plates connected to move with said shaft, asecond shaft connected to rotate with the first shaft and having ahigher coefiicient of expansion than that of the first shaft and anadditional rotor plate on the second shaft capacitatively positioned inrespect to one of said stator plates.

STUART W. SEELEY.

